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Photoinduced Plasmon-Driven Chemistry in trans-1,2bis(4-pyridyl)ethylene Gold Nanosphere Oligomers Emily A. Sprague-Klein, Bogdan Negru, Lindsey R. Madison, Scott C. Coste, Brandon K. Rugg, Alanna M. Felts, Michael O. McAnally, Mayukh Banik, Vartkess A. Apkarian, Michael R. Wasielewski, Mark A. Ratner, Tamar Seideman, George C. Schatz, and Richard P. Van Duyne J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b06347 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 3, 2018

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cation,35-40 sensor design,41-45 metal-organic frameworks (MOFs),46-47 and molecular electronics.38-40 Several recent experiments have identified chemical effects, namely that of charge transfer, from the overall SERS signal using molecular off-resonance excitation.3132 Park and Kim48 utilized molecular off-resonance excitation of gold nanoparticle-4-aminobenzenthiol (ABT)gold film junctions to analyze mode specific on-and-off fluctuation ratios to determine charge transfer enhancement factors. They found that a small percentage of molecules within the junction exhibited charge transfer enhancement in SERS on the order of 101-103, while most molecules remained charge transfer inactive. Additional charge transfer systems have been studied in the SERS signal fluctuations of Au bowtie nanoantennas,49 finite-difference time-domain (FDTD) modeling of the local chemical environment in SERS fluctuations using nanogaps,50 and temporal fluctuations in the tipenhanced Raman scattering (TERS) signal of adenine molecules.51 Other reports have catalogued a range of plasmon-driven chemical behavior52 in nanostructured systems that include light-triggered polymerization,53-54 water splitting,55-56 and photocatalysis.57-60 In this paper, we propose several chemical processes to explain the origins of transient non-resonant molecular spectral activity in plasmonic nanogaps. Other polypyridine molecules, such as 4,4’-bipyridine31 and methyl viologen dichloride32, have been examined previously for their role in plasmon-driven electron transfer reaction. However, extensive consideration of other chemical processes in these highly complex and fluxional nanogap systems has not been addressed. Our work therefore seeks to reinforce the initial observation of a plasmon-driven electron transfer reaction through direct vibrational detection, and to introduce additional chemical processes (photoinduced molecular absorption sitehopping and trans-cis isomerization) that aid in the nuanced understanding of plasmon-driven chemical reactions within nanogaps. EXPERIMENTAL SECTION Single Particle Sample Preparation. Nanosphere oligomer substrates with BPE functionalization were prepared both with and without an outer silica shell. Samples containing a protective silica shell are commercially available (STA Technologies Inc., Albuquerque, NM) and their spectroscopic properties have been previously studied using a range of techniques.61-64 Nanosphere oligomers without a silica shell were fabricated using a methodology that utilizes water soluble polymers as an aggregation stabilizer with the BPE molecules randomly oriented in the nanogap.65 Nanosphere solutions were diluted to single particle concentration and drop-casted onto carbon/formvar TEM grids (Ted Pella, Inc., Redding, CA).63-64 In this way, particles may be easily located and indexed for correlated spectroscopy and imaging studies. Microscope glass coverslips (VWR International, No.1, 25 mm diameter) underwent piranha treatment (3:1 H2SO4:30%H2O2) followed by thorough rinsing with Milli-Q water (18.2 MΩ/cm). Coverslips were then immersed in a base treatment, (5:1:1

H2O:NH4OH:30%H2O2), sonicated for sixty minutes, and again thoroughly rinsed with Milli-Q water. Sample grids were positioned on top of N2 dried coverslips and mounted inside a sample stage above the microscope objective. Single Particle CW Pump-Probe SERS Measurements. During measurements, silica encapsulated particles were imaged using dark-field optical microscopy with a dry condenser and a 40X/0.60 air microscope objective (Nikon Corporation) on an inverted microscope (Nikon Ti-U/E20L80) as shown in Figure S1. Dark-field imaging allows easy identification of single nanoparticle aggregates due to their small metallic cores being great scatters of light, which were imaged using a CCD camera (UNIQ vision, Inc., Santa Clara, CA) attached to an auxiliary microscope port. The two color experimental setup allowed for simultaneous illumination with a 785 nm CW laser (Renishaw Inc.) and 532 nm CW laser (Spectra-Physics, Santa Clara, CA). The 785 nm laser was used to monitor the BPE nanosphere assemblies by collecting low power SERS spectra (power density at the sample = 0.34 µW/µm2), while the 532 nm laser was used for high power illumination (power density at the sample = 118 µW/µm2). A 10:90 beamsplitter (ThorLabs, Newton, NJ) was used to couple the green and red laser beams in such a way that 90% of the green and 10% of the red photons were incident in the microscope focal plane. The experimental setup also features a USB shutter (Picard Industries, Albion, NY) controlled by monitoring the NOT READY output of the PIXIS 400 CCD camera mounted on a Spectra Pro 2500i spectrometer (Princeton Instruments, Trenton, NJ). A shutter allowed for segmented illumination of the BPE nanosphere assemblies and was incorporated into this setup to mitigate the large background caused by the 532 nm laser. The sample was exposed to green light for 10 second intervals between acquisitions with red light. A common sequence for data acquisition consisted of (1) collecting a single frame Raman spectrum at 785 nm for ten seconds, (2) exposing the sample to the 532 nm laser for ten seconds while the camera is not accumulating a spectrum, then (3) collecting a second Raman spectrum at 785 nm for ten seconds. Data acquisition was automated with WinSpec (Princeton Instruments, Trenton, NJ) while the interface to the shutter was controlled using a custom LabVIEW virtual instrument (National Instruments, Austin, Texas). Figure 1b presents a timing diagram depicting the alternation between 532 nm pump and CCD exposure for spectral acquisition. Single particle measurements of nanosphere oligomers without a silica shell were carried out with a 100x oil immersion objective (NA = 0.8–1.3) using an identical microscope setup. SERS spectra with high signal-tonoise ratios were obtained using this optimized setup, which allowed for single particle detection of in-house fabricated nanoparticle oligomers.65 A timing scheme of 1 s pump and 1 s probe was implemented, resulting in increased temporal information compared to our previous work.31 Hundreds of SERS spectra were acquired from thirtysix BPE nanosphere assemblies. More than half of the

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illuminated particles showed spectra containing unexpected fluctuations. These fluctuations are observed in the 700 to 1800 cm-1 fingerprint region of the vibrational spectrum and resemble narrow peaks reminiscent of Raman active lines of vibrational modes. The fluctuations reported here are not broad background oscillations, but rather narrow discrete peaks that are either visible for multiple frames or for single frames collected over tens of seconds. The data sets shown in this report focus on four nanosphere oligomers that expressed transient vibrational modes in the 700 to 1800 cm-1 spectral region that cannot be explained by the neutral normal Raman modes of BPE. All nanosphere oligomers reported in this study exhibited strong reporter molecule signal throughout the course of data acquisition with minimal neutral BPE signal decay across time. Transmission Electron Microscopy (TEM) Imaging. Following spectroscopy, indexed single nanosphere oligomers were correlated and imaged using transmission electron microscopy (TEM). Scans were performed on a Hitachi 8100-TEM using 200 kV of thermionic emission and optimized aperture positions (Condenser = 1, Objective = 2, Selected area = 0). Electrochemical Measurements. Cyclic voltammetry (CV) was measured under a dinitrogen atmosphere at room temperature in an MBraun UniLab Pro glovebox using a CHI 760C potentiostat. A threeelectrode electrochemical cell was used with a 2 mm diameter Pt disk working electrode, a 0.5 mm thick Pt wire counter electrode, and a Ag wire quasi-reference electrode (QRE) referenced to ferrocene/ferrocenium. The analyte was a 10 mM solution of BPE in 10 mL of 0.1 M tetra-n-butylammonium hexafluorophosphate (TBAPF6) in dry, deoxygenated THF. Electron Paramagnetic Resonance Spectroscopy (EPR). The continuous wave electron paramagnetic resonance (EPR) spectrum of the chemically reduced solution phase BPE anion radical was obtained at Xband (~9.8 GHz) frequency using a Bruker Elexsys E680 spectrometer equipped with an ER 4118X-MS5 resonator. Scans were performed with a magnetic field modulation amplitude of 0.5 G and non-saturating microwave power of 0.1 mW. The sample solution was contained in a quartz tube with I.D. 1.50 mm and O.D. 1.80 mm. The experimental spectrum was fit using EasySpin66 v5.2.10 in MATLAB. Visible-NIR Characterization of Neutral & Reduced Species. A spectrophotometer in double beam mode with solvent and baseline/zero correction (Cary 5000, Agilent Technologies) was used to characterize the neutral, singly reduced, and doubly reduced BPE species in solution. Samples were first prepared in a dinitrogen glovebox as described for electrochemical measurements, then transported in sealed quartz cuvettes for spectroscopy measurements. A scanning range of 400 – 1000 nm was used. Chemical Reduction of BPE & Resonance Raman of Reduced Species. BPE was selectively reduced to the anion radical (E½ = −2.186 V vs. Fc/Fc+) by reacting an equimolar amount of BPE in THF with a 0.1 M THF solution of sodium anthracenide; the solution was dark yellow in appearance. BPE was further reduced to

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the dianion (E½ = −2.756 V vs. Fc/Fc+) using an excess of potassium metal in THF followed by filtration through glass wool yielding an opaque purple solution. An automated wavelength scanned measurement was performed using a tunable narrow linewidth continuouswave Ti:Sapphire laser (M Squared, Glasgow United Kingdom). An excitation wavelength of 785 nm was determined to be the optimal wavelength for Raman characterization with minimal background fluorescence while still maintaining resonantly enhanced signal. A 785 nm CW laser (Innovative Photonics) was focused onto the quartz cuvette and Raman scattering was collected in a back-scattering geometry. Collection light was focused into a ½ m imaging spectrograph (Acton SP2500i, Princeton Instruments) with thermoelectrically cooled CCD detector (PIXIS 400BR, Princeton Instruments). Theoretical Calculations. All calculations were performed using the Amsterdam Density Functional (ADF)67 and NWChem68 programs. The triple with polarization (TZP) basis set and the Becke-Perdew (BP86)69-70 exchange correlation functional were used. The BP86 functional was used because it results in harmonic vibrational frequencies that are similar to experimental results without scaling.71 For nitrogen, carbon, and hydrogen, all electrons were modeled while for gold, electrons in atomic orbitals up to the 4d were frozen. For systems that included the gold clusters, relativistic effects were modeled with the zero order regular approximation (ZORA).72-73 Because the oxidized and reduced forms of BPE have an odd number of electrons, calculations of the real and imaginary components of the polarizability tensors were performed using a method described by Aquino and Schatz74 and implemented in the AOResponse module of the NWChem software. For comparison, the closed shell, neutral BPE systems were calculated with the AOResponse modules in both NWChem and ADF software packages,67, 75 following the method described by Jensen et al.76 The AOResponse method uses the short time approximations proposed by Lee and coworkers,77-79 to the Kramers-Heisenberg-Dirac (KHD) formulation of resonance Raman scattering. The wavelength used for these calculations was 785 nm, consistent with the wavelength of the CW laser used in the experiment. The resonance Raman scattering cross section is calculated from the derivative of the isotropic polarizability tensor, αp’as follows: (6.1)

=

( − )

( )

(( / ))

where is the frequency of the incident ﬁeld, and p is the frequency of the pth vibrational mode. The scattering factor, Sp, is a molecular property composed of ! " and #̅ " , which are the isotropic and anisotropic polarizability derivatives. The scattering factor is calculated using: (6.2)


= 45α (") ! ")

+ 7γ

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where the isotropic polarizability derivative is:

! " = ∑/(

(6.3)

-

" // )

and the anisotropic polarizability derivative: (6.4)

# " = ∑/,1 3( )

" " /1 ) ( /1 )

− (

" " // ) ( // )

Following the calculation of the intensity of the Raman scattering for all modes, the spectrum is broadened with a Lorentzian function with a full-width, half-max of 20 cm1 , for easier comparison to experiment.74, 80 EXPERIMENTAL RESULTS

molecular hopping (3) isomerization. Figure 1 displays spectral fluctuations from the 900 – 1700 cm-1 fingerprint region. Time-dependent SERS data are shown in the 2D plots for Figures 1A, 1C, and 1E. The neutral spectra of BPE is present in all frames. Throughout the course of scanning, transient modes appear and disappear that cannot be explained by the normal modes of neutral BPE (see Table S1). In between each acquisition frame (10 seconds each) at 785 nm excitation, the nanosphere oligomers are illuminated with high intensity 532 nm light for 10 seconds at a time. In Figure 1A, transient modes (e.g. 1553 cm-1) are observed beginning in frame 183. Figure 1B compares a neutral SERS spectrum obtained from a BPE nanosphere oligomer before illumination with 532 nm laser light and a SERS spectrum obtained from the same nanosphere assembly after 16.5 total minutes of 532 nm laser light exposure. The spectrum obtained after irradiation with green laser light shows peaks consistent with the vibrational spectrum of surface adsorbed

Figure 1. Independent chemical processes for plasmon-driven chemical behavior. A) 2D plot showing the appearance of transient modes (arrows and highlighted modes) beginning in frame 183 with a prominent peak at 1553 cm-1 B) Top panel shows neutral BPE signal at time zero, middle panel shows an example spectrum containing the transient 1553 cm-1 mode, and bottom panel shows all frames (grey) with time-averaged spectrum (purple) C) 2D plot showing the appearance of transient modes in the 1570 – 1590 cm-1 region beginning in frame 322 D) Top panel shows neutral BPE signal at time zero, middle panel shows an example spectrum containing the transient 1588 cm-1 mode, and bottom panel shows all frames (grey) with time-averaged spectrum (purple) E) 2D plot showing the appearance of transient mode splitting in the 900 – 1300 cm-1 region beginning at frame 68 F) Top panel shows neutral BPE signal at time zero, middle panel shows an example spectrum containing the transient peak splitting at 1222 and 1205 cm-1 and at 1020 and 1040 cm-1, bottom panel shows all frames (grey) with time-averaged spectrum (purple).

Plasmon-Driven Chemistry. Across all four nanosphere oligomers studied, three independent reversible chemical processes of plasmon-driven chemical behavior were identified from transient spectral patterns in three differing wavenumber regions: (1) plasmon-driven electron transfer to form the anion radical species (2)

BPE molecules superimposed with intense peaks at 920, 1103, 1129, 1157, 1234, 1311, 1322, and 1553 cm-1. The bottom panel of Figure 1B shows all of the spectra obtained from a nanosphere oligomer superimposed with the average of all spectra. This average spectrum closely resembles the SERS spectrum of BPE. Similarly, in



Figures 1D and 1F, the time-averaged SERS spectrum resembles BPE suggesting plasmon-driven chemical transformation is restricted to only a small percentage of molecules residing within the ‘hot spot’.31-32, 81 Figures 1C and 1D show time-dependent SERS data for a nanosphere oligomer with an intense peak at 1583 cm-1 and Figures 1E and 1F show a different nanosphere oligomer with peak splitting at 1222 and 1206 cm-1 as well as at 1040 and 1020 cm-1. As seen from Figure 1, the drastic and varied shifts in vibrational energies imply that the chemical processes taking place are transformational, leading to the formation of entirely new chemical species. The formation and consumption of a chemical product on the gold surface can explain the synchronized appearance and disappearance of spectral peaks as seen in Figures S2 and S3 in the Supplementary Information. A change in surface adsorption geometry of the BPE molecules could result in few wavenumber shifts, and it would most likely vary the relative intensity of the BPE spectral peaks,82 but it is unlikely that this would produce the large changes seen in Figure 1. The observed fluctuations are also inconsistent with thermal degradation of BPE, since they manifest as very narrow peaks, as opposed to previously documented broad carbon backgrounds.83 Besides thermal effects, hot electrons emitted from the gold surface can also lead to chemical transformation. In the next sections we analyze evidence for three kinetically favorable plasmon-driven chemical reactions. Chemical Process 1: Plasmon-Driven Electron Transfer. Figure 2 shows the absorption spectra of (a) neutral BPE, (b) singly reduced BPE, and (c) doubly reduced BPE. In the neutral species, no absorption peaks are observed in the visible region whereas for the singly reduced case an absorption peaks are observed at 487, 569, and 694 nm. Absorption peaks for the doubly reduced species are at 543 and 571 nm. Neutral BPE has no absorption peaks within the visible range. In our experiment, the pump field overlaps with the monomer localized surface plasmon resonance for the gold nanosphere oligomer and the probe overlaps with the multicore gold nanosphere resonance.31, 62-63, 65 On a molecular level, the pump overlaps with the 543 nm doubly reduced absorption resonance while the probe overlaps with the broad absorption feature at 694 nm for the singly reduced species. To characterize the energetics and Raman spectra of reduced forms of BPE, we first obtained reduction potentials from a solution phase cyclic voltammogram of a 10

mM solution of BPE (Figure S4). Strong reducing agents were used to generate the singly reduced species and the doubly reduced species using sodium anthracenide and potassium metal reduction, respectively. To confirm the anion radical was generated, an EPR measurement was taken of the singly reduced species and hyperfine coupling values were obtained from simulations (shown in Figure S5 and Table S2). Figure 3A shows SERS spectra of the trans anion radical species for the BPE molecule absorbed on a single nanosphere oligomer along with solution resonance Raman spectra and results from TDDFT calculations. SERS spectra on single nanosphere oligomers are shown with the initial neutral BPE spectrum subtracted out in order to highlight transient modes. The comparisons of theory and experiment are similar to our previously published work for the molecule 4,4’-bipyridine (BPY),31 and they confirm the assignment of the trans anion radical. The SERS data includes results for three different nanosphere oligomers, including two with an encapsulating silica shell and one without a silica shell. Figure S6 shows the time-dependent data set obtained from a nanosphere oligomer (Figure 3A trace c). Initially, in earlier frames, a strong BPE signal was observed (Figures 1A & 1B). The intensity of the BPE signal slowly decreases, and beginning in frame 184 the appearance of new spectral peaks takes place (highlighted in red, Figure S6). The arrows in Figure S7 indicate the concerted appearance and disappearance of some of these new peaks, as well as their persistence from frame to frame. A very intense peak centered around 1550 cm1 appears and disappears, taking place on the order of tens of seconds. Automated spectral analysis techniques have been employed to address the highly diverse and dynamic appearance of these spectral fluctuations (as seen in Figures S2 & S3). Spectrum (d) from Figure 3A shows the solution phase resonance Raman Spectra for the BPE single reduced species, or anion radical. Additional resonance Raman spectra can be found in Figure S8. In the solution phase, the anion radical BPE modes are identified at 1603, 1568, 1428, 1335, 1277, 1246, 1197, 1044, 908, and 872 cm-1. Likewise, for the doubly reduced species the vibrational modes occur at 1649, 1602, 1291, 1235, 1200, 1027, 993, 921, and 907 cm-1. The 1409 cm-1 mode in the spectrum for the singly reduced species coincides with the most intense normal Raman mode of anthracene,84 which is a byproduct from the chemical reduction using sodium anthracenide. Overall, anthracene does not contribute prominently to the Raman signal of the singly reduced BPE species. Good agreement exists between the solution phase anion radical spectrum (singly reduced) species with that shown in Figure 3A for the BPE anion radical spectrum produced through a plasmon-driven electron transfer process in the solidstate. The variance in the peak intensities of anion radical modes on nanosphere oligomers when compared to normal Raman solution measurements can be attributed to differences in molecule-surface interactions such as those that might arise from a changing local potential85 or the excitation of dark plasmon modes.63


Figure 2. Absorption spectra for neutral, singly reduced, and doubly reduced BPE in THF at room temperature. Pump field at 532 nm (green) and probe field at 785

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The BPE anion radical peaks can be assigned to a range of vibrational motions such as pyridine ring distortion modes and quinodal modes coupled to the ethylene stretch (see Table S3 and Table S4). Figure 3c shows a spectral distribution for the 1550 cm-1 mode, where peak distributions have a wider variance for the silica shell case (15 – 20 cm-1 FWHM) when compared to without a silica shell (10 cm-1 FWHM). Changes to the local dielectric host environment for the anion radical due to the presence or lack of silica shell could account for differences in the spectral variance.86-88 Figure 3B displays a TEM image correlated with the SERS spectra shown in

Figure 3. A) Anion radical (singly reduced) BPE spectra across three nanosphere oligomers (a)-(c), resonance Raman spectrum of solution phase singly reduced BPE (d), and gas phase open-shell calculation of the radical BPE anion (e). B) TEM image of nanosphere oligomer correlated to the SERS spectrum (a) shown at left. C) Spectral variance for the 1550 cm-1 mode observed across the three nanosphere oligomers. *indicates residual neutral BPE modes due to imperfect subtraction. 3A (top—spectrum a). Figure

Additional Chemical Processes: Molecular Hopping & Isomerization. Contributions from two additional processes, molecular hopping and isomerization, that are observed in BPE nanosphere oligomers can be found in Figure 4. The various species have been identified through corroboration with open-shell TDDFT calculations and vibrational mode assignments can be found in Table S5 and Table S6. Here we see a peak near 1580 cm-1 in (a)-(d), which contrasts with the absence of a peak in Figure 4 (e) and (f) showing that the TDDFT spectra associated with two binding states on a Au25 gold cluster (vertex and face, as pictured) are in good agreement with the experimental spectral intensity near 1580 cm-1. This contrasts with the unbound or symmetrically bound case (see Figure S9), where the 1580 cm-1 mode is not present in TDDFT calculations. Thus the 1580 cm-1 vibrational mode appears only when symmetry breaking occurs, in other words when the BPE molecule is bound at one end to either a face or vertex site on the Au25 cluster. Another feature of the spectra

near 1580 cm-1 in Figure 4A is flipping behavior that can be understood as a transition between the two binding states on a gold cluster where both spectra (a) and (d) appear to show the BPE molecule binding to a vertex site whereas spectra (b) and (c) show face-binding. Figure 4B also shows similar flipping behavior between two isomer states, although the spectral regions involved are quite different compared to Figure 4A. Spectral doublets occurring at 1222 and 1205 cm-1 as well as at 1022 and 1044 cm-1 are the defining features for a configurational change from the trans- to neutral cis-isomer. Figure S10 shows comparison between experimental spectra and TDDFT calculations for the combined effect of isomerization and plasmon-driven electron transfer to form the cis-anion. Defining features of the cis-anion include triplet peaks at 1454, 1429, and 1407 cm-1, in addition to peaks at 1315, 1285, 1213, and 1016 cm-1. The relative contributions of each chemical process as well as their macroscopic kinetic timescales are addressed in the following section. DISCUSSION Generation of Chemical Product & Surface Electrons. The novel pathway to photochemistry on metal surfaces is provided through metal-molecule interfacial states, whereby a metal electron can be promoted to electronically excited states of the molecule that can undergo chemistry. The spectral signatures for the anion radical created through a plasmon-driven electron transfer reaction are reproducible, with peaks appearing in multiple frames on the seconds timescale. Of the new modes that appear, the 1551 cm-1 and 1432 cm-1 modes are highly reproducible while modes near 957 and 1303 cm-1 regions also appear in multiple frames. Comparisons of experiment and calculations of the anion radical yield excellent agreement, especially in the 1450 – 1550 cm-1 region, where no other source of vibrational peaks could be identified. A negatively charged state of BPE would cause significant shifts of the BPE Raman modes towards lower frequencies, which has previously been observed for other molecular systems, such as TTF,89 TCNQ,90 and most recently BPY-h8/d831 absorbed on gold nanosphere oligomers. Observation of the anion radical is possible only if the charged species undergoes significant enhancement within the ‘hot spot’ junction. It is known that the most enhancing sites in SERS contribute to 24% of the overall SERS intensity while making up only 0.006% of total sites.81 Observation of the blinking behavior of BPE anion radical signal is plausible and can be attributed to formation and decay of the anion radicals and to diffusion into and out of highly enhancing sites on the nanosphere assemblies. The intensity fluctuation of the new spectral features does not correlate with intensity fluctuations of the BPE signal, which is further indication that only a very small percentage of BPE molecules undergo chemical change. Recent studies suggest that plasmon-driven electron transfer is essentially a single molecule event, affecting only a tiny fraction of molecules localized or ‘trapped’ in a highly enhancing nanogap region.31-32 The 532 nm generated electrons interact with surface adsorbed BPE molecules and activate the ensuing

ACS Paragon Plus Environment Figure 4. Schematic of additional chemical processes. A) Varied molecular absorption from face- to vertex-binding and B) trans-cis isomerization.


chemistry and reactive decay of BPE molecules. Electrons that trigger the ensuing chemistry can be generated at the gold surface via two processes: (1) Upon radiation at 532 nm, the LSPR mode of gold monomers and to a lesser extent91-92 interband transition of the gold metal are excited and result in the release of hot electrons from the metal surface. The low energy requirements of plasmon resonances have resulted in many recently published reports that focus on plasmon driven chemical processes.93-98 Plasmon excited nanoparticles have been shown to be an efficient source of hot electrons92, 99-100 when the plasmon quantum excites an electron from the Fermi level of the metal to a nearby electron acceptor.101 In this way, hot electrons can transfer into electronic states of adsorbed species with high quantum efficiencies.102 (2) Additionally, transient reflecting grating spectroscopy studies employing a 1kHz ultrafast laser determined that most chemisorbed crystal violet molecules received charge transfer electrons from the metal upon excitation with green laser light close in energy to the interband transition of gold.103 The charge transfer was possibly due to the excitation of valence electrons residing close to the Fermi level to adsorption energy states of the molecules.103

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which is similar in mode character to the 1630 cm-1 in neutral BPE. A similar shift is observed going from the 1300 cm-1 neutral BPE mode to the 1240 cm-1 anion radical BPE Kekulé mode. The 1028 cm-1 anion radical modes exhibits ring distortion via asymmetric bond stretching. A similar shift is observed for the anion radical of the cis configuration of BPE as seen previously in Figure S10. As shown in the molecular orbital diagram in Figure 5, the LUMO of the anion radical has antibonding character for the ethylene C=C bond and certain bonds of the pyridine rings, leading to the reduced frequencies for many modes. Our gas phase electron affinity calculations reveal that the BPE anion has a stabilization energy of 1.5 eV when compared to neutral BPE, which means that the BPE anion will not undergo degradation after electron transfer. Furthermore, while BPE molecules are not resonantly enhanced at 785 nm, the BPE anion radical is resonantly enhanced, resulting in single molecule sensitivity and the feasibility of detecting single reduction events. The formation of a single BPE anion radical would not cause a detectable jump in the BPE signal intensity, but it would lead to the addition of new peaks corresponding to the anion radical, peaks with intensities comparable to the BPE signal. The small size of the ‘hot spot,’ small number of BPE anion radicals, and surface diffusion of these anion radical species to and from the hot spot leads to the observed intermittent detection of peaks corresponding to the BPE anion radical. Vibrational Mode Shifting & Charge Accumulation. Systematic time-dependent spectral shifts were observed in nanosphere oligomers undergoing a plasmondriven electron transfer reaction. Figure 6 shows data from two different nanosphere oligomers in which the SERS spectra for the anion radical is observed, along with a systematic Stark shift of the vibrational modes. The 1550 cm-1 mode, assigned to the anion radical quinodal modes, shifts 13 cm-1 after eighteen successive

Figure 5. The resonant Raman scattering spectra comparing the neutral and anion radical forms of trans conformers of BPE upon excitation with 785 nm light.

The theoretical resonance Raman spectra of the transBPE conformer of the anion radical shows significant spectral differences compared to the neutral form as demonstrated in Figure 5. The stretching vibration of the weaker ethylene bond in the anion radical of BPE also couples to the quinoidal like ring distortion at 1550 cm-1

Figure 6. A) Vibrational mode shifting in a nanosphere oligomer with correlated TEM image B) Additional shifting in a second nanosphere oligomer.

intervals of pump ‘on’ and ‘off’. Similarly, the 1588 cm-1 assigned to molecule-surface binding, shifts 7 cm-1 after eight successive intervals of pumping time. Previous studies have demonstrated that molecules are capable of directly measuring the modulation of local electromagnetic fields due to electron tunneling in plasmonic nanogaps.104-105 The resulting time-dependent vibrational shifts are due to a buildup of charge due to optical rectification in strongly coupled nanoparticle systems. A


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recent study on charge buildup of Au nanoparticles concluded that steady-state light excitation can lead to photoelectrochemical potentials as high as 240 mV.106 With this in mind, it is possible for the silica coating of the nanosphere assemblies to insulate against charge buildup on the gold surface. Future experiments will test this hypothesis by monitoring the LSPR of individual nanosphere assemblies under high power green laser light illumination.107 The BPE anion radical signal intensity flickers in and out within the time resolution of the experiments carried within, making it impossible to distinguish between electron recombination, photothermal desorption, surface diffusion, and irreversible degradation. It is, however, possible that the flicker in intensity of the anion radical signal is due to the presence of a single anion radical within the ‘hot spot.’ This situation would result in large intensity fluctuations depending on the location within the ‘hot spot,’ as well as relative fluctuations in intensity between different modes of the BPE anion radical.82 Future experiments will aim at increasing the time resolution of the experiment, so that we may produce kinetic models for the BPE anion radical signal loss. Chemical Timescales & Relative Contribution By Process. A timeline of chemical reaction dynamics for the three operative processes can be seen in Figure S11. Figure S11 shows the timeline of first appearance of anion radical modes in both silica coated and uncoated gold nanosphere oligomers. The first anion radical modes appear after 3 minutes of optical pump time in particles encapsulated with a silica shell whereas the delay time to first appearance of anion radical modes is much shorter in oligomers with no silica shell. However, signal stability of the anion radical is pronounceably lower and more erratic in oligomers with no silica shell. In comparison, molecular site hopping is a delayed process which is not photoinitiated immediately and persists for many frames afterwards (3 – 6 minutes of optical pumping). Isomerization processes occur on a relatively shorter timescale, after 170 s of pump time. Because each spectrum is collected on the seconds timescale, the chemical products must be long-lived. Reconciling the long-lived chemical reaction dynamics with the shortlived hot electron dynamics is still an area requiring further understanding,32, 108 and future studies are currently underway to investigate reaction intermediates on increasingly shorter timescales. Figure 7 shows the distribution of events by chemical process and the relative contributions of the three effects in explaining the spectral fluctuations in single nanosphere oligomer SERS. Automated peak fitting in MATLAB was employed to extract spectral locations for all modes in our timedependent SERS data. In our analysis, one count corresponds to the observance of a vibrational peak in one frame and does not refer to peak intensity information. Figure 7A shows frequency locations for peaks corresponding to the neutral BPE molecule only. Figure 7B shows all the transient modes arising from an intense 532 nm pump field with spectral information from the neutral BPE molecule subtracted. An enlarged version of the transient mode histogram can be seen in Figure 7C. Several spectra regions can be assigned to the coexistence of multiple chemical processes, as indicated

by additional symbols. A total of 3612 transient spectral modes were collected across four differing nanosphere oligomers in the 900 – 1700 cm-1 fingerprint region. Out of that total, we have identified and assigned a chemical process (trans anion radical, molecular binding, cis isomerization, and the cis anion radical) for spectral regions corresponding to 2075 of those transient modes. Counts due to contribution from residual neutral BPE signal amounts to 914 spectral counts. As a result, our mechanistic analysis can account for 84% (2989 out of 3612) of all transient spectral features in the timedependent SERS data. Decomposition of BPE into moderately stable molecular species was also considered, however, none of the theoretically calculated modes can account for the transient spectral modes observed in our SERS data set (Figure S14). The remaining spectral features could be explained by the excitation of field gradients, which has been addressed in Supporting Figure S15. CONCLUSION In this study, we investigate highly complex spectral fluctuations present in CW pump-probe SERS data on single nanosphere oligomers. Vibrational signatures for the anion radical are observed for BPE, a molecule for which the anion radical spectrum is previously unknown. The existence of the BPE anion radical is confirmed using TDDFT calculations of the harmonic vibrations and resonance Raman scattering cross-sections. Additionally, the BPE anion radical was generated in solution using strong chemical reducing agents, and probed using both resonance Raman and CW EPR measurements to further substantiate the vibrational signature for the anion radical in the solid state. In addition to the formation of an anion radical species, other chemical processes were considered, allowing us to account for 84% of observed transient spectral behavior and chemical transformations occurring within strongly coupled nanogaps. Applica-

Figure 7. A) Neutral BPE counts from time-dependent SERS measurements, aggregated across four distinct nanosphere oligomers B) Counts from spectral fluctuations across four distinct nanosphere oligomers with neutral BPE subtracted C) Enlarged version of histogram shown in (B). Green histogram bars represent molecular binding peaks, red represents trans ACS Paragon Plus Environment anion radical, blue represents cis anion radical, purple represents cis isomer, and grey represents unidentified. Symbols represent the following: *residual neutral BPE normal modes; α shared modes between molecular binding and the trans- and


tions of these findings have the potential to greatly impact all areas of organic molecular electronics, photocatalytic nanoreactors, and photovoltaic design. Future studies will aim to increase the temporal resolution of the experiment to gain knowledge about chemical intermediates as well as probe the spectral range of these lighttriggered plasmon-driven chemical reactions.

ASSOCIATED CONTENT Supporting Information This material is available free of charge via the Internet at http://pubs.acs.org. Experimental schematic and electronics timing diagram, Automated peak fitting analysis, Vibrational mode assignments using TDDFT, Solution phase cyclic voltammogram and EPR trace, Hyperfine values, Bimodal distribution of anion radical modes, Kinetic models of neutral BPE signal loss, Calculated photodegradation products, Field gradient analysis of spectral patterns

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Present Addresses ||

B.N.: Department of Chemistry, Sonoma State University, Rohnert Park, CA 94928.

ξ

L.R.M.: Department of Chemistry, University of Washington, Seattle, WA 98195. Γ

M.O.M.: Institute for Defense Analyses, Alexandria, VA 22311.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. S.C.C. and B.K.R. contributed equally.

ACKNOWLEDGMENT The authors would like to thank Eero Hulkko, Michael Mattei, Tyler Ueltschi, and Yue Wu for helpful discussions. E.A.S.-K., B.N., M.O.M., A.M.F., M.B., V.A.A., G.C.S., and R.P.V.D. were supported by the National Science Foundation Center for Chemical Innovation dedicated to Chemistry at the Space-Time Limit (CaSTL) Grant CHE-1414466. E.A.S.-K., L.R.M., and M.O.M. acknowledge support from the National Science Foundation Graduate Research Fellowship Program (DGE-0824162). T.S. acknowledges support from the National Science Foundation Grant CHE1465201. B.N. and R.P.V.D. acknowledge support from the National Science Foundation (CHE-1506683). M.R.W. acknowledges support from the National Science foundation under grant no. CHE-1565925.

ABBREVIATIONS CW, continuous wave; SERS, Surface-enhanced Raman Spectroscopy; BPE, trans-1,2-bis-(4-pyridyl)-ethylene; LSPR, Localized surface plasmon resonance; TDDFT, Time-dependent Density Functional Theory; TEM, Transmission Electron Microscopy; EPR, Electron Paramagnetic Resonance

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